Add indexed option for custom field B-tree indexes

[Glacier-Luo]

Problem

EmDash lets you define custom fields on collections, but there's no way to add a database index on those fields. For projects with non-trivial data, this is a gap — queries that filter or sort by a custom field hit a full table scan.

In my own project (running on PostgreSQL), the ability to index custom fields is essential. Without it I've been stuck — I can define the fields I need but can't make the queries against them efficient or correct at scale.

Proposal

Add an indexed: boolean option to field definitions. When enabled, a standard B-tree index (CREATE INDEX IF NOT EXISTS idx_{table}_{column}) is created on the field's column. The index is dropped when the option is toggled off or the field is deleted.

• Works on both SQLite and PostgreSQL (standard SQL, no dialect-specific syntax)
• Exposed in the Admin UI as a toggle on the field editor
• Supported in seed files (indexed?: boolean on SeedField)
• Separate from unique (which already creates a partial unique index) — indexed is a plain non-unique B-tree

Implementation

I have a working implementation in PR #1512 (being split per maintainer request). The core pieces:

• Migration adds indexed column to _emdash_fields
• SchemaRegistry.createField / updateField call createPlainIndex / dropFieldIndexes as needed
• Admin field editor gets an "Indexed" toggle
• Seed apply passes indexed through to createField